One of the most significant challenges facing directed-energy systems and imaging technologies operated on airborne platforms is the problem of propagating light through an optically active flow field such as an airplane""s turbulent boundary layer. The emerging wavefronts are further distorted by atmospheric turbulence and require correction through the use of an adaptive optical system.
In order to correct for the optical aberrations, the wavefront distortions need to be measured at a high temporal bandwidth, and current optical wavefront sensors are limited in speed specifically at the charge-coupled device (CCD) sensor array.
The invention described herein is an innovative, low-cost technology that significantly increases the temporal bandwidth of conventional optical wavefront sensors by replacing the CCD array with an array of position-sensing devices (PSD), or lateral-effect detectors, described below.
The application of adaptive optical systems to imaging and/or atmospheric propagation problems requires the measurement of optical wavefront distortions prior to correction. Optical wavefront distortions are measured using an optical wavefront sensor, and corrections are applied using a deformable mirror. The sensing and correction process involves a feedback control loop. As with most control problems, it is desirable to sample the optical wavefront at approximately ten times the deformable mirror""s cutoff frequency.
The frequency at which a deformable mirror needs to be actuated to correct for transmission through atmospheric turbulence (assumed isotropic and homogeneous) can be estimated by the Greenwood frequency. Ground-based observatories have Greenwood frequencies on the order of 200 Hz, and consequently wavefront sensing requirements, being an order of magnitude higher, are on the order of 2 kHz.
Optical propagation through turbulence from airborne platforms puts more demands on adaptive optical systems, with Greenwood frequencies on the order of 2 kHz and wavefront sensor requirements approaching 20 kHz. The high temporal bandwidth requirement of 20 kHz is not feasible with current-generation optical wavefront sensor technology. As a result, there is a need to develop optical wavefront sensors that can operate at significantly higher temporal bandwidths.
Two of the most commonly used optical wavefront sensor technologies are based on the following methodsxe2x80x94AC lateral shearing interferometers and Shack-Hartmann wavefront sensors. Each requires the interrogation of an array of detectors. The shearing interferometer is frequently coupled to an array of photodiodes, whereas the Shack-Hartmann utilizes an array of quad cells or a CCD array. Of the two methods, the Shack-Hartmann is the most advantageous in that it is able to resolve wavefront distortions without encountering the 2xcfx80 phase ambiguity problem of an interferometric device. Consequently Shack-Hartmann wavefront sensors are often the preferred sensing method for adaptive optics applications. The main disadvantage to the Shack-Hartmann wavefront sensor is the fact that in order to sense the off-axis position of each subaperture""s focal spot it is necessary to interrogate a large number of pixels. Thus, the sensor""s temporal bandwidth is limited.
The Shack-Hartmann wavefront sensor consists of an array of lenses (lenslet array) fabricated from a single piece of glass or plastic. Each lens or subaperture, if square, forms a 2-dimensional sinc2 diffraction pattern in the focal plane (commonly referred to as subaperture spot). The aperture size and focal length, which is the same for each lens, is designed to achieve the required resolution and dynamic range of a wavefront slope measurement.
In some of the early Shack-Hartmann wavefront sensor designs, an array of quad cells was used to sense the off-axis position of the diffraction spot from each subaperture. The use of quad cells limits sensor performance in two ways. First, a quad cell can only sense deflections less than or equal to the subaperture spot""s diffraction-limited radius. This condition results when the subaperture spot has moved entirely to one side or the other of the quad cell, and further motion would result in no further change in detector signal (i.e., signal saturation).
A second limitation results from the fact that the quad-cell output signal is sensitive to the shape of the subaperture spot, leading to errors under conditions of irregular object shape. In order to circumvent these problems, quad cells were replaced by a CCD array.
A Shack-Hartmann wavefront sensor constructed using a CCD array avoids the problems encountered using quad cells by imaging each subaperture onto a Nxc3x97N sub-grid of pixels. By choosing a large number of pixels per subaperture, spot deflections larger than the spot""s diffraction radius can be measured. The use of center-of-gravity-based centroiding schemes enables accurate determination of deflections even when the irradiance distribution or spot shape from a subaperture is irregular due to higher-order aberrations. These two advantages have led to the use of CCD arrays in place of quad-cell arrays.
Depending on bandwidth, dynamic range and accuracy requirements, anywhere between 5xc3x975 to 10xc3x9710 pixel arrays have been used per subaperture. Current-generation, commercially available, CCD-array cameras operate at frame rates approaching 1 kHz, and next-generation research cameras have been described with continuous frame rates as high as 10 kHz. As higher frame rates are considered, the benefits of using a CCD array begin to be offset by the increased number of signals that need to be acquired and processed per second. Higher frame rates have shorter exposure times, leading to a reduction in the overall signal irradiance per pixel. Higher frame rates also result in an increase in analog-to-digital (A/D) converter noise or read noise. Both effects tend to reduce the camera""s signal-to-noise ratio (SNR). Consequently, although higher data rate cameras continue to appear on the market, the increased load imposed on data processing schemes limits temporal bandwidth and accuracy of Shack-Hartmann wavefront sensor designs using CCD arrays.
Shack-Hartmann wavefront sensor technology involves a simple optical configuration that is easy to understand and integrate into a system and, as a result, has become well established. Consequently, technology development that builds on the strengths of the Shack-Hartmann design are appealing from both a technical and marketing perspective.
Shack-Hartmann designs using arrays of quad cells are deficient due to limited dynamic range and spot-shape sensitivity. Shack-Hartmann sensor designs using CCD arrays suffer from the large amount of data that needs to be read out and processed, as well as a reduction in SNR as sensor frame rate is increased. A sensor that combines the analog processing speed of the quad cell with the dynamic range and spot-shape independence of the CCD array results in a Shack-Hartmann sensor design with significant improvements in temporal bandwidth. An improved Shack-Hartmann sensor design with these qualities is described herein.
A distant relative of the quad cell is the lateral-effect detector, a position-sensing device. The lateral-effect detector is a large area photodiode, consisting of a square sensing area bounded by four strip terminals. Two independent currents are generated in orthogonal directions between terminals on opposite sides of the active area. When exposed to photons, the lateral-effect detector induces a current flow laterally away from the location of the incident beam. The conductance of the path between the location of the beam and each strip terminal is related to the distance between beam and terminal; therefore greater currents are generated by a beam that is closer to a terminal. Consequently, the current measured from each strip terminal may be used to determine the beam""s position on the sensor surface. This results in a voltage signal that is proportional to beam location.
The advantage of a lateral-effect detector over a quad cell is that the signal does not saturate for beam deflections greater than the diffraction spot radius. A second advantage is that the resulting centroidal position is not sensitive to the shape of the incident beam. The lateral-effect detector, like the quad cell, has an advantage over CCD-array-based sensors as fewer signals need to be measured per subaperture, and centroiding operations are performed in analog, reducing data-processing requirements. Thus, it can be seen that the lateral-effect detector possesses some of the best features of both the quad cell and the CCD array.
Construction of a Shack-Hartmann wavefront sensor using an array of lateral-effect detectors is attractive from both a temporal-bandwidth standpoint and from an economic perspective. Considering a 20xc3x9720 wavefront-sensor example, replacing the CCD array with an array of lateral-effect detectors results in a reduction in the number of signals to be acquired by a factor of 32. Furthermore, with all of the centroiding calculations performed in analog, data-processing requirements are reduced and logic-circuit construction greatly simplified.
An additional benefit exists as continuous-exposure sensors do not have the signal-to-noise problems associated with the short-integration times of high-clock-rate CCD arrays. It is evident that use of these low-cost, commercially available sensor systems makes the final sensor design very economical, while at the same time solving a large portion of the temporal bandwidth constraints and accuracy limitations associated with current Shack-Hartmann sensor designs.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the drawings.